1. Field of the Invention
The present invention pertains to the cross-section configuration of a spiral groove in a mandrel surface of an annular die and the method of producing the groove configuration. More specifically, the present invention pertains to the cross-section configuration of a spiral groove cut into a mandrel surface of an annular die where the groove configuration approaches an “almost round” cross-section configuration and the method of cutting the groove.
2. Description of the Related Art
In the production of blown film where a hot liquid polymer melt stream is distributed around a cylindrical gap inside a die prior to being blown from the die to produce a tubular film, at least one mandrel is often employed in the die where the mandrel has a spiral groove formed in a cylindrical exterior surface or a cylindrical interior surface of the mandrel. The hot liquid melt is delivered through the spiral groove to distribute the melt around the cylindrical gap inside the die prior to being blown from the die. Examples of such mandrel constructions are disclosed in FIG. 3 of the Cole U.S. Pat. No. 4,201,532, incorporated herein by reference and FIG. 1 of the Gates U.S. Pat. No. 5,261,805, also incorporated herein by reference.
A blown film die can be comprised of several coaxial cylindrical mandrels assembled together on a common base. In this type of die construction typically more than one of the mandrels has a spiral groove. An example of such a blown film die is disclosed in FIG. 1 of the Mahler U.S. Pat. No. 7,811,073, incorporated herein by reference.
The spiral grooves on the mandrel surfaces are typically fed from a hot liquid melt stream that flows through a number of feeders or ports that each lead to a spiral groove. The feeders or ports can be located upstream of the mandrel or can be formed on the mandrel itself. The feeders or ports can be machined along the exterior surface of the mandrel as shown in FIG. 1 of the Mahler, et al. patent or can be machined through the mandrel in an encapsulated construction as shown in FIG. 1 of the Cole patent. In addition, the feeders or ports may feed more than one spiral in a “Y” configuration as shown in FIG. 1 of the Mahler, et al. patent.
The spiral geometry is typically designed where the depth and width of the spiral decreases as the spiral extends from its feeder or port around the mandrel surface and toward the discharge end of the blown film die. At the same time, the axial spacing between adjacent spirals increases as the spirals extend around the mandrel surface and toward the discharge end of the blown film die.
The conventional cross-sectional shape or configuration of a mandrel spiral is usually semi-circular or “D” shaped. This is shown in drawing FIG. 1 and FIG. 1 of the Cole patent. Additionally, the spiral groove can be cut in the cylindrical exterior surface of a mandrel as shown in FIG. 1 of the Mahler et al. patent, the cylindrical interior surface of a mandrel as shown in FIG. 1 of the Mahler et al. patent, or both as shown in FIGS. 3-5 of the U.S. Patent Publication of Stommel No. US2004/0166192, incorporated herein by reference. This is also shown in FIG. 1 of the Mahler, et al. patent.